Blood and skin DC reconstitution shall be investigated in patients affected by hematological diseases and receiving HLA-matched, mobilized peripheral blood stem cells from related or unrelated donors. We plan to simultaneously collect and process double biopsies, i.e. from graft versus host disease affected and non-lesional skin of patients at onset of acute graft versus host disease following allogeneic hematopoietic stem cell transplantation. Emigration assay, Chimerism analysis and immunohistochemistry will be performed.